1. Field of the Invention
The present invention is related to a miniature microstrip antenna, and especially to a microstrip antenna of which the size is reduced but the antenna still keeps the broadband function in all the directions.
2. Description of the Prior Art
Coil antennae with spiral structures are widely used in various communication instruments; the most common ones are mobile phones. Basically, such coil antennae are exposed. In the recent years however, notebook style computers with communication function adopt built-in microstrip antennae.
Earlier microstrip antennae, such as those disclosed in the U.S. Pat. Nos. 3,921,177 and 3,810,183, are normally made from round or rectangular thin metallic sheets, they are separated from ground sheets with filled-in dielectric material, such microstrip antennae generally only suit narrower bandwidths. U.S. application Ser. No. 07/695686 , now abandoned, provides a polygonal spiral microstrip antenna having improvement on the earlier microstrip antennae, the bandwidth thereof approaches a normal antenna with a predetermined impedance. However, such a spiral microstrip antenna has the defect that, when in the condition of low frequency, the diameter of the antenna will be quite enlarged such that it will not suit modern communication instruments carried on one's own.
To this, Taiwan patent No. 81108896 (U.S. application Ser. No. 07/798700), now abandoned, provides a reduced and broadband microstrip antenna, but a spiral antenna element is provided on a separated ground sheet, and a kind of dielectric material and load of predetermined thickness are interposed therebetween, so that size of the whole antenna can hardly be further reduced.